The invention relates to a continuously variable transmission and a working machine comprising such a continuously variable transmission.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders and articulated haulers. Although the invention will be described hereinafter with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as dump trucks, graders, excavators or other construction equipment.
A working machine is provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load. A working machine may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines and similar environments.
A wheel loader is usually provided with a prime mover, such as an internal combustion engine, a transmission line with a torque converter and a gearbox for supplying power to the driving wheels of the wheel loader. In addition to supply power to the driving wheels, the internal combustion engine has to supply power to one or more hydraulic pumps of a hydraulic system of the wheel loader. Such a hydraulic system is used for lifting operations and/or steering the wheel loader.
Hydraulic working cylinders are arranged for lifting and lowering a lifting arm unit, on which a bucket, forks or other type of attachment or working tool is mounted. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front body part and a rear body part of the wheel loader which body parts are pivotally connected relative to each other.
The gearbox can be a continuously variable transmission (CVT) having a variator unit which gives a lot of advantages compared to a stepped gearbox. For example, the velocity of the working machine can be controlled independently of the speed of rotation of the prime mover. If the continuously variable transmission has a speed of rotation range comprising a mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input, shaft of the gearbox at the same time as torque can be transmitted from the input shaft to the output shaft (usually referred to as an infinitely variable transmission (IVT) with geared neutral), the torque converter traditionally used in working machines can be omitted. By the expression “zero or close to zero” is meant a speed of rotation of the output shaft which is zero or in the size of a few rotations per minute.
By the geared neutral function the operation of the hydraulic system can also be separated from the propulsion of the working machine. Furthermore, the hydraulic system can be driven by the prime mover when the working machine stands still without using any clutch for disengagement of the prime mover relative to the transmission line.
The continuously variable transmission may have a planetary gear wheel unit enabling the power transmitted to be split between the variator unit and the planetary gear wheel unit. By such a gearbox having a powersplit (PS), in many operation modes only a minor part of the power has to be transmitted by the variator unit. The remaining power is transmitted by the planetary gear wheel unit. This implies an improved overall efficiency since the loss of energy is considerably smaller with respect to the planetary gear wheel unit compared to the variator unit. Furthermore, the size of the variator unit can be reduced.
A prior art continuously variable transmission is disclosed in U.S. Pat. No. 5,403,241. The gearbox described in U.S. Pat. No. 5,403,241 has a variator unit and a planetary gear wheel unit. A disadvantage with this gearbox is that the speed ratio range will be limited in some of the powersplit modes. For each mode, the speed ratio range depends on one or more basic gear ratios of the planetary gear wheel unit. For the mentioned powersplit modes the speed ratio range depends on one basic gear ratio. Here, the speed ratio range will increase with a lower absolute value of this basic gear ratio. The lowest possible absolute value of this basic gear ratio is, however, limited by geometrical conditions. The gear ratio is defined by the ratio between the diameter of the ring wheel and the diameter of the sun wheel. The absolute value of the basic gear ratio can be decreased to some extent by reducing the size of the planet wheels, but due to the fact that the planet wheels have to be designed with a smallest allowed diameter the lowest possible absolute value of the basic gear ratio is limited, and, thus the speed ratio range will be limited.
It is desirable to provide a continuously variable transmission defined by way of introduction, which transmission enables an increased speed ratio range in a powersplit mode of the transmission.
In a continuously variable transmission according to an aspect of the invention, the basic gear ratio of the planetary gear wheel unit is defined by the ratio between the diameter of the second sun wheel and the diameter of the first sun wheel. This implies that the geometrical limitation with respect to the size of the planet wheels associated with the prior art transmission is eliminated, and that the lowest possible absolute value of the basic gear ratio can be decreased and the speed ratio range can be increased accordingly.
This in turn gives the possibility to use a transmission with reduced number of powersplit modes in for example certain wheel loader applications where a reduced number of powersplit modes is desired and still provide the requisite speed ratio range.
In addition, the transmission according to the invention is cost effective, since one planet carrier and one ring wheel can be eliminated, and there is no need to connect two planet carriers to the input shaft or to connect a sun wheel of a first unit to a ring wheel of a second unit which ring wheel in turns to be connected to an output shaft. The invention also enables a more compact design, at least in the axial direction of the planetary gear wheel unit.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.